Guard cell-specific upregulation of sucrose synthase 3 reveals that the role of sucrose in stomatal function is primarily energetic.

Isoform 3 of sucrose synthase (SUS3) is highly expressed in guard cells; however, the precise function of SUS3 in this cell type remains to be elucidated. Here, we characterized transgenic Nicotiana tabacum plants overexpressing SUS3 under the control of the stomatal-specific KST1 promoter, and investigated the changes in guard cell metabolism during the dark to light transition. Guard cell-specific SUS3 overexpression led to increased SUS activity, stomatal aperture, stomatal conductance, transpiration rate, net photosynthetic rate and growth. Although only minor changes were observed in the metabolite profile in whole leaves, an increased fructose level and decreased organic acid levels and sucrose to fructose ratio were observed in guard cells of transgenic lines. Furthermore, guard cell sucrose content was lower during light-induced stomatal opening. In a complementary approach, we incubated guard cell-enriched epidermal fragments in (13) C-NaHCO3 and followed the redistribution of label during dark to light transitions; this revealed increased labeling in metabolites of, or associated with, the tricarboxylic acid cycle. The results suggest that sucrose breakdown is a mechanism to provide substrate for the provision of organic acids for respiration, and imply that manipulation of guard cell metabolism may represent an effective strategy for plant growth improvement.

Tobacco guard cells fix CO2 by both Rubisco and PEPcase while sucrose acts as a substrate during light-induced stomatal opening.

Transcriptomic and proteomic studies have improved our knowledge of guard cell function; however, metabolic changes in guard cells remain relatively poorly understood. Here we analysed metabolic changes in guard cell-enriched epidermal fragments from tobacco during light-induced stomatal opening. Increases in sucrose, glucose and fructose were observed during light-induced stomatal opening in the presence of sucrose in the medium while no changes in starch were observed, suggesting that the elevated fructose and glucose levels were a consequence of sucrose rather than starch breakdown. Conversely, reduction in sucrose was observed during light- plus potassium-induced stomatal opening. Concomitant with the decrease in sucrose, we observed an increase in the level as well as in the (13) C enrichment in metabolites of, or associated with, the tricarboxylic acid cycle following incubation of the guard cell-enriched preparations in (13) C-labelled bicarbonate. Collectively, the results obtained support the hypothesis that sucrose is catabolized within guard cells in order to provide carbon skeletons for organic acid production. Furthermore, they provide a qualitative demonstration that CO2 fixation occurs both via ribulose-1,5-biphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPcase). The combined data are discussed with respect to current models of guard cell metabolism and function.